1. Field of the Invention
The invention relates to throttle control system and method for an internal combustion engine.
2. Description of the Related Art
As is known in the field of the art, at extremely low temperature, so-called blow-by gas that contains much water after flowing through the passages of a PCV system (Positive Crankcase Ventilation System) causes “icing” at a throttle valve which has been cooled down by low temperature intake air. Specifically, when the blow-by gas passes through the throttle valve, the water contained therein is frozen between the throttle valve and an internal wall of a throttle bore. In view of this, Japanese Patent No. 3189717 provides a throttle control system that executes a particular procedure for determining whether a throttle motor is locked when icing occurs at the throttle valve.
More specifically, when the ambient temperature is lower than a specific temperature below which the above-mentioned throttle icing is likely to occur, this throttle control system extends an observation time that is taken before determining locking-up of the throttle motor after the locking-up has been first detected. As a result, it is possible to avoid determining locking-up of the throttle motor when the throttle motor is locked up due to icing which will typically last only for a limited time. That is, the throttle control system determines locking-up of the throttle motor only when the throttle motor is locked up due to jammed gears, or the like, which normally will not be resolved in time.
Besides, Japanese Patent No. 3458935 proposes increasing a control value when the difference between an actual throttle opening and a target throttle opening is large in order to bring the actual throttle opening to the target throttle opening quickly.
As is known, a throttle valve is exposed to water, oil, and various extraneous matters, and they may seize up the throttle valve temporarily under some conditions. In particular, at low temperature, water and oil contained in blow-by gas from a known PCV system or EGR gas from a known EGR system (Exhaust Gas Recirculation system) may form some ice and tar between the throttle valve and the inner wall of the intake passage, which seize up the throttle valve.
Also, with a conventional throttle valve made of metal such as aluminum, it is possible to prevent throttle icing by having warm water passages, for example. However, with a resin throttle valve that is now increasingly used, having such warm water passages is difficult in design. Also, the low heat capacity of such a resin valve further increases the difficulty in prevent icing at low temperature.